a. Field of the Invention
Broadly speaking, this invention relates to the interrogation and identification of fixed or moving targets. More particularly, in a preferred embodiment, this invention relates to methods and apparatus for interrogating moving vehicles by illuminating a transponder fastened to the vehicle with a beam of microwave energy.
B. Discussion of the Prior Art
In recent years commercial trucking firms have been subject to numerous hijackings. For example, in New York City alone there is an average of about one hijacking a day. As a result of this and other problems, various state and federal authorities have begun to search for some reliable means for quickly determining when a hijacking has occurred so that police helicopters or the like may be dispatched to locate, and uniquely identify, the stolen vehicle in the dense traffic environment in which such hijackings typically occur.
Clearly, it would be highly advantageous if the stolen vehicle could be identified from the air, although interrogation from the ground would also be desirable. This requirement suggests the use of a transponder which would be placed on, or imbedded in, the roof of the truck or its trailer. Ideally, such a transponder would be inexpensive, tamper-proof, operable during the day or night and under all weather conditions, and have the capability of generating a coded response such that the interrogated vehicle could be uniquely and positively identified.
An article entitled "DINADE -- A New Interrogation, Navigation and Detection System" which appeared in Microwave Journal, Vol. X, No. 6, May 1967 at pp 70, 72, 75, 77 and 78 discloses a technique wherein the vehicle to be identified is provided with a target comprising a plurality of serially connected diodes. When these diodes are illuminated by an interrogating beam of microwave energy at a fundamental frequency, fo, the non-linear characteristics of the diodes cause the generation of harmonics of the fundamental frequency, which harmonics are then reradiated back towards, and detected by, the interrogator. By appropriately biasing the diodes, analog or digital information may be modulated onto the harmonic signal thus permitting the interrogator to uniquely identify the particular transponder radiating the harmonic signal.
Unfortunately, the modulation and coding scheme proposed in the above publication suffers from the disadvantage that it will be difficult to decide if the coded harmonic signal returned from the transponder is, in fact, the coded signal corresponding to the transponder on a stolen vehicle. This is due to the relatively unsophisticated coding technique proposed in the aforesaid article, which technique cannot successfully discriminate between the cluttered signals which would be received from the large number of transponders which, of necessity, must be expected in any dense urban area if the system is to be successful.